Re: Do different type of fishes(their flesh) have different pH value?

Thanks for the interesting question Chiew Lin,

Yes. The flesh (meat) of different fish species does have different pH values. However, as far as I can tell, those pH values should be uniformly lower than neutral pH (7.0), and not higher (pH 8). Lets take a look at the underlying biochemistry, so that you can see why.

The term pH comes to us from France. It is an abbreviation of a French term, puissance de Hydrogen, which refers to the molar concentration of hydrogen ions (protons) in a water solution. Acidic solutions have high concentrations of protons, while basic solutions have very low proton concentrations. Puissance in French means power in English, so pH is a measure of the power (exponent) of the value of the molar concentration of protons in an aqueous solution. In particular, pH is the negative log of the molar concentration of hydrogen ions (protons) in an aqueous solution. So, if an acidic solution has a proton concentration of 10^-4 M, it has a pH of 4. A basic solution has a proton concentration of 10^-9 M, so it has a pH of 9. A solution of pure water molecules will spontaneously ionize into H+ and OH- ions at a low frequency; the concentration of protons in that case will be 10^-7 M. This concentration is considered to be neutral, and the pH is 7.

Most organisms have pHs that are around neutral values. Our bodies maintain this optimal pH through the use of buffers, so that if you drink a really sour (acidic) lemonade, your internal pH remains close to 7. If a cell’s pH strays too far from 7, then its proteins and DNA will start to denature, and the cell will die.

Now, lets look at fish. Fish are very muscular animals, and their muscles have to do alot of work when they swim, requiring alot of energy. Like fish, we use oxygen to generate energy; our cells oxidize glucose to carbon dioxide (CO2) and water (H2O) in a process called aerobic respiration. In the absence of oxygen, many organisms use a process called anaerobic respiration or fermentation to convert glucose into lactic acid (lactate) or ethyl alcohol (ethanol). Anaerobic respiration does not generate as much energy as aerobic respiration, but it generates enough to keep a cell going for a while. This is why your muscles ache when you excercise hard -- the muscle cells produce lactic acid when they can’t get enough oxygen. Later, when you have time to relax, you can get more oxygen into your tissues and the lactic acid is oxidized to CO2 and H2O, or even back into glucose, and the muscle aches stop.

When an animal is killed for meat, its major body functions cease long before the cellular functions cease. The animal stops breathing and its blood stops pumping, but the muscle cells are still alive. They try to stay alive by oxidizing glucose, but since the lungs or gills are no longer functioning to provide oxygen to the blood, and since the heart is no longer pumping the blood, those cells can only carry out anaerobic respiration, and they generate lots of lactic acid.

As I’m sure you’ve noticed, lactic acid is an acid. This means that it will lower the pH of an aqueous solution when added. As the body dies, its ability to buffer acids decreases as well, and the overall pH of the muscle tissue decreases because the cells generate new lactic acid. The degree to which this process progresses is determined in part by genetics; some fish species will generate more lactic acid after death than others. In addition, the treatment of an animal can affect the degree to which it generates lactic acid after it dies. If an animal is starved before it dies, there will not be as much glucose available to convert to lactic acid after its death. Finally, the size of the fish can contribute to the degree of pH change. Large fish which swim to exhaustion will have a greater immediate decrease in muscle pH than small fish, because they can generate more lactic acid.

So, overall I think you can see that there are a variety of factors that will contribute to the pH of a piece of fish. Genetics, as well as the circumstances and manner in which the fish was caught will result in a variety of different pH values.

Here is a table from the United States Food and Drug Administration describing the pH values of a few different fish species.

Fish (most fresh) : 6.6 - 6.8

Tuna fish : 5.2 - 6.1

Whitefish : 5.5

Sturgeon : 5.5 - 6.0

Salmon : 6.1 - 6.3

Herring : 6.1 - 6.4

Freshwater (most) : 6.9 - 7.3

Now, as for why your class got consistent readings of pH 8, I don’t really have much of an idea. I don’t know what technique you were using to evaluate the pH value. If you were simply holding a piece of litmus paper against the piece of fish it would be difficult to tell the difference between a pH of 6 and a pH of 8. On the otherhand, the fish you were using might have been preserved in some way that made it more basic.

References

Texas A&M University has a great meat science division that has all sorts of interesting meat subjects. In addition, I think you might find the University of Guelph Department of Animal and Poultry Science’s page on converting muscle into meat interesting. Although these pages focus on cattle and pigs, the lessons can also be applied to fish.